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ZeroS: Zero‑Sum Linear Attention for Efficient Transformers
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Proposes ZeroS, a linear-time attention that removes the constant 1/t zero-order softmax term and reweights the zero-sum residuals with learned gates, enabling signed weights and contrastive operations.
Problem
Linear attention is O(N) but typically underperforms softmax attention; convex-combination weights only permit additive blending and accumulate uniform bias that dilutes attention in long contexts.
Approach
Subtract the uniform 1/t zero-order softmax term and the first-order deviation, then gate the resulting zero-sum residuals with learned sigmoid gates and multiply by a signed cosine angular term. Prefix-sum scans of three d x d states make this linear time while allowing negative weights.
Key results
- On MAD, ZeroS reaches 73.5 average versus 74.5 for softmax Transformer, and ZeroS-SM reaches 75.4.
- On WikiText-103, ZeroS obtains 24.61 test perplexity versus 24.78 for the Transformer, and ZeroS-SM obtains 24.17.
- On ImageNet-1k with DeiT-Tiny, ZeroS reaches 75.51% top-1 versus 72.20% for DeiT.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Reintroduce the 0-th order softmax term (ablation) | ZeroS · WikiText-103 | PPL: 23.91 -> 24.05 (val); 24.61 -> 24.74 (test) | Hurt |
| Reintroduce the 0-th order softmax term (ablation) | ZeroS · MAD | Average: 73.5 -> 69.4 | Hurt |
| Replace reweighted zero-sum softmax with standard softmax (ablation) | ZeroS · MAD | Average: 73.5 -> 67.6 | Hurt |
| Replace reweighted zero-sum softmax with standard softmax (ablation) | ZeroS · WikiText-103 | PPL: 23.91 -> 24.21 (val); 24.61 -> 24.97 (test) | Hurt |
| Remove gating (ablation) | ZeroS · MAD | Average: 73.5 -> 70.8 | Hurt |
| Remove normalization (ablation) | ZeroS · MAD | Average: 73.5 -> 69.4 | Hurt |
| Remove the angular component (ablation) | ZeroS · MAD | Average: 73.5 -> 42.2 | Hurt |
| Remove positional embeddings from the angular component (ablation) | ZeroS · MAD | Average: 73.5 -> 46.1 | Hurt |
| Use additive positional embeddings in the angular component (ablation) | ZeroS · MAD | Average: 73.5 -> 69.0 | Hurt |
| Remove the radial component (ablation) | ZeroS · MAD | Average: 73.5 -> 65.9 | Hurt |
| Use alternative radial scoring (linear projection, quadratic form, 2-distance, or averaging) (ablation) | ZeroS · MAD | Average: 73.5 -> 70.0; 70.8; 71.1; 73.0 | Hurt |
| Replace causal linear ZeroS with zero-sum reweighting on standard softmax attention | ZeroS · MAD | Average: 73.5 -> 75.4 | Helped |
Surprising findings
- Reintroducing the 0-th order softmax term degraded performance on MAD and WikiText-103, contrary to the expectation that the constant term aids stability.
Limitations
- No fused CUDA kernel; implementation invokes an existing linear-attention scan three times.
- No large-scale LLM training or LLM benchmark evaluation due to resource constraints.
- Evaluation focuses primarily on autoregressive tasks; non-causal tasks left for future work.
Experimental setup
Datasets: MAD MQAR RegBench WikiText-103 OpenWebText2 ImageNet-1k Weather Solar ETT
Benchmarks: MAD MQAR RegBench WikiText-103 OpenWebText2 ImageNet-1k time series forecasting benchmarks
Metrics: accuracy perplexity top-1 accuracy MSE MAE latency throughput memory
Baselines: Hyena Mamba GLA DeltaNet LinAttn Transformer FLASH cosFormer HGRN2 GatedDeltaNet Mamba2 iTransformer
Builds on: linear attention RoPE Taylor expansion of softmax Differential Transformer
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